System for and method of detecting a buried conductor

ABSTRACT

A system for detecting a buried conductor comprises a transmitter for producing an alternating test current in the buried conductor and a receiver for detecting an electromagnetic field produced by the test current in the buried conductor. A communication link is provided between the receiver and the transmitter. The receiver measures the signal to noise ratio, SNR, of the electromagnetic field produced by the test current and if the SNR is below a threshold then the receiver controls the transmitter to set frequency and/or power characteristics of the test current.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to United Kingdom Patent ApplicationNo. GB 0803874.7, filed on Feb. 29, 2008, and entitled “System for andMethod of Detecting a Buried Conductor,” the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a system for and method of detecting aburied conductor.

BACKGROUND OF THE INVENTION

Before commencing excavation or other work where electrical cables,fibre optic cables or other utilities ducts or pipes are buried, it isimportant to determine the location of such buried cables or pipes toensure that they are not damaged during the work. Once a buried utilityis located the depth of the utility can be calculated to determine asafe excavation depth.

Current carrying conductors emit electromagnetic radiation which can bedetected by an electrical antenna. If fibre optic cables or non-metallicutilities ducts or pipes are fitted with a small electrical tracer line,an alternating electrical current can be induced in the tracer linewhich in turn radiates electromagnetic radiation. It is known to usedetectors to detect the electromagnetic field emitted by conductorscarrying alternating current.

One type of such detector works in one of two modes, namely ‘active’ or‘passive’ modes. Each mode has its own frequency bands of detection.

The passive mode comprises ‘power’ mode and ‘radio’ mode. In power mode,the detector detects the magnetic field produced by a conductor carryingan AC mains power supply at 50/60 Hz, or the magnetic field re-radiatedfrom a conductor as a result of a nearby cable carrying AC power,together with higher harmonics up to about 5 KHz. In radio mode, thedetector detects very low frequency (VLF) radio energy which isre-radiated by buried conductors. The source of the original VLF radiosignals is a plurality of VLF long wave transmitters, both commercialand military.

In the active mode, a signal transmitter produces an alternatingmagnetic field of known frequency and modulation, which induces acurrent in a nearby buried conductor. The signal transmitter may bedirectly connected to the conductor or, where direct connection accessis not possible, a signal transmitter may be placed near to the buriedconductor and a signal may be induced in the conductor. The buriedconductor re-radiates the signal produced by the signal transmitter.

A number of factors must be considered when using the active mode. Asthe transmitter is conventionally powered by on-board batteries it isimportant to efficiently generate the test signal whilst conserving thepower expended by the transmitter as much as possible so as to prolongthe battery life of the transmitter. Therefore the power of a detectabletest signal emitted by the transmitter should be minimised to reducebattery consumption. In addition, a high power signal can couple tounwanted lines and spread over the lines, making it difficult to detectthe target buried conductor.

The transmitter can be configured to transmit an alternating test signalat a number of frequencies. The choice of frequency depends on a numberof factors, for example the ease of inducing the test signal into theburied conductor and interference from ambient signals.

Regarding the choice of frequency of the alternating test signal, a highfrequency signal is used when the line impedance is high (typically ifthe ground is dry or when the target wire is an insulated twisted pairwithout a common ground reference), a medium frequency signal istypically used for mains power supply cables and continuous metal pipesand a low frequency signal is used for long distance tracing where agood earth return is provided at the cable end.

The frequency of an initially chosen test signal may not be suitable dueto interference from ambient signals. Signals being carried by othernearby conductors at the same frequency or having a harmonic frequencythe same as the frequency of the test signal may lead to a poor signalto noise ratio of the signal detected at the receiver. Interference dueto such ambient frequencies may require altering the frequency of thetest signal produced by the transmitter to avoid interference by theambient frequencies.

Therefore, when using a transmitter to produce an alternating testcurrent in the buried conductor the operator may be required toiteratively set the transmitter signal power and frequency so that thesignal produced by the transmitter is of a suitable frequency to bedetected by the receiver and of an efficient power. This requires theparticipation of a separate operator for the transmitter and receiver orthe operator of the receiver to repeatedly travel between thetransmitter and the target site where the receiver is located, which istime consuming.

When applying a test signal to a target buried conductor to be traced adifficulty can arise if there is a second buried conductor in closeproximity to the target buried conductor. The field radiated by thetarget buried conductor carrying the test signal may induce a current inthe second buried conductor due to capacitive coupling or direct bondingbetween the two buried conductors as the second conductor carries aground return current. The induced current in the second conductor isthen re-radiated by the second conductor and may be picked-up by thereceiver. Therefore when tracing the route of a buried conductor it isnecessary to verify that the conductor that is being traced is thetarget conductor and not a second buried conductor onto which the testsignal is coupled from the target conductor.

U.S. Pat. No. 5,260,659, assigned to Radiodetection Limited on its faceand the contents of which are incorporated herein by reference,describes a system for tracing a buried current carrying conductor. Analternating test signal having first and second components, related infrequency and phase, is applied to the target conductor and theelectromagnetic field is detected at a plurality of positions. Byconsidering the phase of the first and second components a decision canbe made about whether the conductor being detected is the targetconductor or a second conductor onto which the test signal has beenburied.

In this application we describe an improved system for detecting aburied conductor which overcomes some of the disadvantages ofconventional systems.

SUMMARY OF THE INVENTION

Embodiments of the present invention advantageously provide a system forand method of detecting a buried conductor.

According to a first aspect of the invention there is provided a systemfor detecting a buried conductor, the system comprising: means forproducing an alternating test current in said buried conductor; meansfor detecting an electromagnetic field produced by the test current insaid buried conductor; and means for providing a communication linkbetween the means for detecting an electromagnetic field and the meansfor producing an alternating test current; wherein the means forproducing an alternating test current and the means for detecting anelectromagnetic field are configured for the means for detecting anelectromagnetic field to set, via the communication link between themeans for detecting an electromagnetic field and the means for producingan alternating test current, characteristics of the test currentproduced by the means for producing an alternating test current inresponse to the electromagnetic field detected at the means fordetecting an electromagnetic field.

The means for producing an alternating test current and the means fordetecting an electromagnetic field may be configured for the means fordetecting an electromagnetic field to set characteristics of the testcurrent produced by the means for producing an alternating test currentby: the means for detecting an electromagnetic field being configured todetermine desirable characteristics of the test current and to transmitthe desirable characteristics of the test current to the means forproducing an alternating test current; and the means for producing analternating test current being configured to receive the desirablecharacteristics of the test current and set the characteristics of thetest current to correspond to the desirable characteristics determinedby the means for detecting an electromagnetic field.

The characteristics of the test current set by the means for detectingan electromagnetic field may comprise power and/or frequency of the testcurrent.

The communication link between the means for detecting anelectromagnetic field and the means for producing an alternating testcurrent may be provided by a transceiver at each of the means forproducing an alternating test current and the means for detecting anelectromagnetic field.

The communication link between the means for detecting anelectromagnetic field and the means for producing an alternating testcurrent may be a duplex or half-duplex communication link.

The means for detecting an electromagnetic field may comprise means forcalculating the signal to noise ratio, SNR, of the detectedelectromagnetic field at a specified frequency range.

The means for detecting an electromagnetic field may be configured toalter the frequency of the alternating test current, for example byincrementing or decrementing the frequency by 17 Hz, if the SNR is belowa lower threshold.

The lower threshold may be 20 dB in a 10 Hz bandwidth, preferably 12 dBin a 10 Hz bandwidth and preferably 6 dB in a 10 Hz bandwidth.

The means for detecting an electromagnetic field may be configured toalter the power of the test current if the SNR is above an upperthreshold. The upper threshold may be 40 dB in a 10 Hz bandwidth,preferably 50 dB in a 10 Hz bandwidth and preferably 60 dB in a 10 Hzbandwidth.

The means for detecting an electromagnetic field may be configured toset the characteristics of the test current based on the compleximpedance of the ground at the transmitter.

The communication link between the means for producing an alternatingtest current and the means for detecting an electromagnetic field may bea wireless communication link. The wireless communication link may usethe Bluetooth communication protocol.

The means for detecting an electromagnetic field may be configured toset characteristics of the test current produced by the transmitterwithout intervention from an operator of the system.

The test current in said buried conductor may be produced by the meansfor producing an alternating test current by means of an output module,the output module either radiating an electromagnetic field to inducethe test current in said buried conductor, being directly connected to apart of said buried conductor or clamping the output module around saidburied conductor.

The means for detecting an electromagnetic field may comprise aplurality of antennas for detecting the electromagnetic field producedby the test current in said buried conductor.

Each antenna may output a field strength signal representative of theelectromagnetic field at the antenna.

The system may further comprise amplifiers arranged to amplify the fieldstrength signals and analogue to digital converters to convert the fieldstrength signals into digital signals; and a digital signal processorarranged to process the digital signals and to isolate signals ofpredetermined frequency bands.

According to a second aspect of the invention there is provided a methodof detecting a buried conductor, the method comprising: providing atransmitter for producing an alternating test current in said buriedconductor; providing a receiver for detecting an electromagnetic fieldproduced by the test current in said buried conductor; providing acommunication link between the receiver and the transmitter; and thereceiver setting characteristics of the test current produced by thetransmitter via the communication link between the receiver andtransmitter.

The receiver may set the characteristics of the test current produced bythe transmitter by: determining desirable characteristics of the testcurrent at the receiver; transmitting the desirable characteristics ofthe test current from the transmitter; receiving the desirablecharacteristics of the test current at the receiver; and setting at thetransmitter the characteristics of the test current to correspond to thedesirable characteristics determined at the receiver.

The characteristics of the test current set by the receiver may comprisepower and/or frequency of the test current.

The communication link between the receiver and transmitter may beprovided by a transceiver at each of the transmitter and receiver.

The communication link between the receiver and transmitter may be aduplex or half-duplex communication link.

The receiver may calculate the signal to noise ratio, SNR, of thedetected electromagnetic field at a specified frequency range.

The receiver may alter the frequency of the alternating test current,for example by incrementing or decrementing the frequency by 17 Hz, ifthe SNR is below a lower threshold.

The lower threshold may be 20 dB in a 10 Hz bandwidth, preferably 12 dBin a 10 Hz bandwidth and preferably 6 dB in a 10 Hz bandwidth.

The receiver may alter the power of the test current if the SNR is abovean upper threshold. The upper threshold may be 40 dB in a 10 Hzbandwidth, preferably 50 dB in a 10 Hz bandwidth and preferably 60 dB ina 10 Hz bandwidth.

The characteristics of the test current may be set by the receiver basedon the complex impedance of the ground at the transmitter.

The communication link between the transmitter and receiver may be awireless communication link. The wireless communication link may use theBluetooth communication protocol.

The characteristics of the test current produced by the transmitter maybe set by the receiver without intervention from an operator of thesystem.

The test current in said buried conductor may be produced by thetransmitter by means of an output module, the output module eitherradiating an electromagnetic field to induce the test current in saidburied conductor, being directly connected to a part of said buriedconductor or clamping the output module around said buried conductor.

The receiver may comprise a plurality of antennas for detecting theelectromagnetic field produced by the test current in said buriedconductor.

Each antenna outputs a field strength signal representative of theelectromagnetic field at the antenna. The receiver may further compriseamplifiers arranged to amplify the field strength signals and analogueto digital converters to convert the field strength signals into digitalsignals; and a digital signal processor arranged to process the digitalsignals and to isolate signals of predetermined frequency bands.

According to a third aspect of the invention there is provided a carriermedium carrying computer readable code for controlling a microprocessorto carry out the method described above.

According to a further aspect of the invention there is provided asystem for detecting a buried conductor, the system comprising: atransmitter for producing an alternating test current in said buriedconductor; a receiver for detecting an electromagnetic field produced bythe test current in said buried conductor; and means for providing acommunication link between the receiver and transmitter; wherein thetransmitter and receiver are configured for the receiver to set, via thecommunication link between the receiver and transmitter, characteristicsof the test current produced by the transmitter in response to theelectromagnetic field detected at the receiver.

There has thus been outlined, rather broadly, certain embodiments of theinvention in order that the detailed description thereof herein may bebetter understood, and in order that the present contribution to the artmay be better appreciated. There are, of course, additional embodimentsof the invention that will be described below and which will form thesubject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of embodiments inaddition to those described and of being practiced and carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein, as well as the abstract, are for thepurpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a system for detecting a buriedconductor according to an embodiment of the invention.

FIG. 2 is a block diagram of the transmitter of the system of FIG. 1.

FIG. 3 is a block diagram of the receiver of the system of FIG. 1.

FIG. 4 is a flow chart of a method of setting the transmit power andfrequency of the test signal produced by the transmitter using thesystem of FIG. 1 in a first embodiment of the invention.

FIG. 5 is a flow chart of a method of setting the transmit power andfrequency of the test signal produced by the transmitter using thesystem of FIG. 1 in a second embodiment of the invention.

FIG. 6 is a flow chart of a method of resetting a phase differencebetween different frequency components of the test signal produced bythe transmitter using the system of FIG. 1 in an embodiment of theinvention.

DETAILED DESCRIPTION

The invention will now be described with reference to the drawingfigures, in which like reference numerals refer to like partsthroughout.

FIG. 1 is a schematic representation of a system 1 for detecting aburied conductor according to an embodiment of the invention, comprisinga portable transmitter 5 and a portable receiver 7. The transmitter 5 isplaced in proximity to a buried conductor 3 and acts as a means forproducing an alternating current test signal in the buried conductor 3.Thus, the receiver 7 acts as a means for detecting the electromagneticfield 11 produced by the test current in said buried conductor 3.

An aerial in the transmitter is fed with an AC voltage to produce amagnetic field 9 which links around the buried conductor 3, therebyinducing an alternating current test signal in the buried conductor 3.The alternating current test signal is radiated as a electromagneticfield 11 by the buried conductor 3 which can be detected by the receiver7.

Both the transmitter 5 and receiver 7 comprise a communications module13, 15. Each communications module 13, 15 comprises a transceiver thatprovide a communication link between the receiver 7 and the transmitter5. Control signals are transmitted using a wireless communicationstechnique using the Bluetooth (RTM) standard. In other embodiments otherwired or wireless techniques may be used to transmit control signalsbetween the receiver 7 and the transmitter 5.

FIG. 2 is a block diagram of the portable transmitter 5 of the system 1of FIG. 1. The alternating current test signal is radiated by an outputmodule 21 and coupled into the buried conductor 3 to produce thealternating test current in the buried conductor 3. In other embodimentswhere direct access to the conductor is available the transmitter signalmay be applied to the buried conductor 3 by conventional techniques ofdirectly connecting an output module 21 to the buried conductor 3 or byclamping the output module 21 around the buried conductor 3. The outputmodule 21 may also flood an area with a signal which energises allconductor lines in the area.

The test signal produced by the output module 21 is controlled by asignal processor module 23. The signal processor module 23 sets thepower, frequency and modulation scheme of the signal to be applied tothe buried conductor 3. The signal processor module 23 and output module21 are controlled by a controller 25. The operation of the transmitter 5is set either by an operator via a user interface module 27 or by thecommands received at the communications module 15 sent from the receiver7, as described below.

The user interface module 27 conveys information to the operator of thetransmitter 5 and may comprise one or more of a display for displayinginformation to the operator of the device, input devices such as akeypad or a touch sensitive screen and an audible output devices such asa speaker or beeper. In addition to the communications module 15 sendingand receiving commands to/from the communications module 13 of thereceiver 7, the communications module 15 also enables the transmitter 5to be connected to a personal computer (PC) or a personal digitalassistant (PDA) (not shown). The transmitter 5 further comprises amemory module 29 and a power supply unit (PSU) 31 comprising a powersource such as batteries and power management circuitry.

The transmitter 5 comprises means for calculating the complex impedanceof the ground at the transmitter 5. The complex impedance of the groundis measured by comparing the phase and magnitude of the voltage drivingthe output module 21 with the phase and magnitude of the current throughthe output module 21. The relationship between these phases depends onthe nature of the load (the utility) to which the test signal isapplied. If the load is dominantly resistive then the current andvoltage will be substantially in phase. For a dominantly capacitive loadthe current will lead the voltage at a phase angle up to 90° and if theload is dominantly inductive then the current will lag the voltage by aphase angle up to 90°. The components of the portable transmitter 5 arehoused in a housing (not shown).

FIG. 3 is a block diagram of the portable receiver 7 of the system 1 ofFIG. 1. An electromagnetic field 11 radiated by the buried conductor 3is detected by antennas in an antenna module 31. Each antenna outputs afield strength signal representative of the electromagnetic field at theantenna. The outputs from the antenna module 31 are fed into a signalprocessor module 33 which comprises a signal processor module 33 forisolating signals of a desired frequency or frequencies and processesthese signals to derive their characteristics. The signal processormodule 33 comprises a pre-amplification stage for amplifying the fieldstrength signals output from the antennas if the detected signal isweak. The signal processor module 33 further comprises an analogue todigital converter for converting the field strength signals into digitalsignals and a digital signal processor block for processing thedigitised signals. Like the transmitter 5, the receiver 7 also comprisesa controller 35, PSU 37, communications module 13, memory 39 and userinterface 41. The components of the portable receiver 7 are housed in ahousing (not shown).

The communications modules 13, 15 of the receiver 7 and transmitter 5provide a communication/data link between the receiver 7 and transmitter5 which enhances the locating experience of the operator of the system1, simplifies the operator interface and facilitates single useroperation of the transmitter 5 and receiver 7. In this embodiment thecommunication link is a radio frequency telemetry system providinghalf-duplex communication between the transmitter 5 and receiver 7. Inother embodiments a full duplex communication link may be used.

By using a long range Bluetooth® transceiver, such as the Ezurio® BTM404long range Blutetooth® Serial Modules, the communication link betweenthe transmitter 5 and receiver 7 may be maintained up to a line of sightrange of 800 m. This communication standard provides a good balancebetween the range of the communication link and low power consumptionrequired from the batteries of the transmitter 5 and receiver 7 tomaintain the communication link. Alternative communication standards maybe used in other embodiments.

In this embodiment the receiver 7 takes full-authority control of thetransmitter 5. The communication transport layer is based on a standardslip protocol suitable for asynchronous and synchronous serial data. Thereceiver 7 acts as the bus master and the transmitter 5 as a slave. Allcommands sent from the receiver 7 to the transmitter 5 are acknowledgedby the transmitter 5 to allow the transmitter 5 and receiver 7 to besynchronised. In the event of a checksum error or an acknowledge signalnot being received by the receiver 7 both receiver 7 and transmitter 5assume the command to be inactive.

In a first embodiment of the system 1 receiver commands and transmitterresponses are given in Table 1.

TABLE 1 Receiver Commands and Transmitter Responses Command SetFrequency Set Power Demand Set Voltage Demand Set Current DemandIncrement/Decrement Frequency Induction Mode ON/OFF 8KFF ON/OFF CDWaveform ON/OFF ACD Waveform ON/OFF Increment CD F1 Phase

In a second embodiment the receiver 7 and transmitter 5 have an expandedcommand and response set as shown in Tables 2.

TABLE 2 Additional Receiver Commands and Transmitter Responses of the2^(nd) embodiment Command Request Ground Impedance Request Power OutputRequest Voltage Output Request Current Output Request Battery Volts

In the first embodiment of the invention the system 1 of FIG. 1 isconfigured to remotely set characteristics of the test signaltransmitted by the transmitter 5 according to the method shown in FIG.4.

The transmitter 5 and receiver 7 are turned on and in step S101 thecommunication link between the transmitter 5 and receiver 7 isestablished. The characteristics of the test signal comprise itsfrequency and power and these are initially set at the transmitter 5 viaits user interface 27 at step S103. The test signal is transmitted bythe transmitter 5 and coupled to the buried conductor, either directlyor indirectly to produce an alternating test current in the buriedconductor 3. The frequency of the test signal is input to the receiver 7so that the receiver 7 monitors signals at the frequency of the testsignal produced by the transmitter 5. At step S105 the receiver 7detects the electromagnetic field 11 radiated by the buried conductor atthe test signal's frequency using known amplification, filtering andsignal processing techniques.

At step S107 the receiver 7 calculates the signal to noise ratio (SNR)of the test signal at the test signal's frequency. The SNR of the testsignal should be above a lower threshold level to be able to process thetest signal and at step S109 the receiver 7 determines if the signaldetected at the test signal's frequency is above the lower thresholdlevel. If there is noise at the test signal's frequency, for example dueto interference from ambient signals or harmonics thereof at the samefrequency, then at step S111 the receiver 7 determines a new test signalfrequency by nudging the frequency a small amount, for example by ±17Hz. The decision to nudge the locate frequency may either be takenautomatically by the receiver (based on an assessment of the SNR) or amanual operation at the request of the operator. At step S113 thereceiver 7 sends a “Set Frequency” and a “Set Power Demand” command tothe transmitter 5 which is acknowledged by the transmitter 5 by sendingan acknowledge response to the receiver 7. The method then returns tostep S103 and repeats steps S103 to S113 until the SNR of the testsignal detected at the receiver 7 is above the lower threshold level.

The lower threshold level is at least 20 dB (10 Hz bandwidth),preferably at least 12 dB (10 Hz bandwidth) and further preferably atleast 6 dB (10 Hz bandwidth).

Once the SNR is determined to be above the lower threshold level then atstep S115 the receiver 7 determines if the SNR of a magnitude such thatthe transmit power of the test signal transmitted by the transmitter 5can be reduced. An upper threshold level is at 40 dB (10 Hz bandwidth),preferably 50 dB (10 Hz bandwidth) and further preferably at least 60 dB(10 Hz bandwidth). Reducing the transmit power of the test signalreduces the power consumption of the transmitter 7, thereby enhancingthe operating time of the PSU 29 of the transmitter 7 whilst stillproducing a test signal with an acceptable SNR at the receiver 7. If alower SNR can be tolerated then at step S117 the receiver 7 determines alower test signal power and at step S113 the receiver 7 sends a “SetFrequency” and a “Set Power Demand” command to the transmitter which isacknowledged by the transmitter 5 by sending an acknowledge response tothe receiver 7. The method then returns to step S103 and repeats stepsS103 to S117 until the SNR of the test signal detected at the receiver 7is above the lower threshold level and the transmit power of the testsignal is at an optimal level. Once these conditions are met then atstep S119 the receiver processes the test signal to determine, forexample, the depth of the buried conductor.

In the second embodiment of the invention the system 1 of FIG. 1 isconfigured to remotely set properties of the test signal transmitted bythe transmitter 5 according to the method shown in FIG. 5. As statedabove with reference to Tables 3 and 4, in the second embodiment thereceiver 7 and transmitter 5 have an expanded command and response set.

As for the first embodiment, in the second embodiment the transmitter 5and receiver 7 are turned on and the communication link between thetransmitter 5 and receiver 7 is established in step S201. At step S203the transmitter 5 measures the complex impedance of the ground and sendsthis value to the receiver. In this embodiment this step is in responseto the “Request Ground Impedance” command sent by the receiver 7 to thetransmitter 5. In other embodiments the transmitter 6 may be configuredto periodically measure and send the measurement of the ground's compleximpedance to the receiver 5 or the transmitter may be configured tomeasure and send the measurement of the ground's complex impedance tothe receiver 5 once the communication link between the transmitter 5 andreceiver 7 are established.

The receiver 7 uses the complex impedance measurement received from thetransmitter 5 to initially set the frequency of the transmitted testsignal. If the load is determined to be of low resistance or dominantlyinductive then a low frequency is initially set for the transmitter'stest signal. If the load is of high resistance or dominantly capacitivethen a high frequency is initially set for the transmitter's testsignal.

At step S205 the receiver 7 sends a “Set Frequency” and a “Set PowerDemand” command to the transmitter 5 which is acknowledged by thetransmitter 5 by sending an acknowledge response to the receiver 7. Thepower and frequency of the test signal are set by the transmitter 5 inaccordance with the command sent by the receiver 7 at step S207. Thetest signal is transmitted by the transmitter 5 and coupled to theburied conductor 3.

At step S209 the receiver 7 detects the electromagnetic signal 11radiated by the buried conductor 3 and at step S211 the receiver 7calculates the signal to noise ratio (SNR) of the test signal at thetest signal's frequency as for the first embodiment. At step S213 thereceiver 7 determines if the signal detected at the test signal'sfrequency is above the threshold level. If the SNR is below thethreshold then at step S215 the receiver 7 determines a new test signalfrequency by nudging the frequency by a small amount as described above.The method then returns to step S205 and repeats steps S205 to S215until the SNR of the test signal detected at the receiver 7 is above thethreshold level.

Once the SNR is determined to be above the threshold level then at stepS217 the receiver 7 determines if the SNR is of a magnitude such thatthe transmit power of the test signal transmitted by the transmitter 5can be reduced. If a lower SNR can be tolerated then at step S219 thereceiver 7 determines a lower test signal power. The method then returnsto step S205 and repeats steps S205 to S219 until the SNR of the testsignal detected at the receiver 7 is above the threshold level and thetransmit power of the test signal is at an optimal level. Once theseconditions are met then at step S221 the receiver 7 processes the testsignal to determine, for example, the depth of the buried conductor 3.

The techniques described in U.S. Pat. No. 5,260,659 are used in thepresent invention in combination with the communication link between thetransmitter 5 and receiver 7 to enhance the detecting experience of theoperator. In this embodiment one signal component f₁ is twice thefrequency of the other signal component f₂. In other embodiments onesignal component may be an even integer multiple or a sub-harmonic ofthe other.

If a test signal applied to a target conductor is coupled to a secondproximal conductor then it is likely that this signal will be phasereversed with respect to the original signal. The coupling mechanismbetween adjacent conductors may be either resistive, capacitive orinductive. In the case of a resistive coupling there is still likely tobe a phase shift as the signal will return to the transmitter on thepath of least impedance. Therefore, with a knowledge of the phaseφ_(f1), φ_(f2) of the components f₁, f₂ of the detected signal it ispossible to discriminate between the outgoing ‘original signal’ and theunwanted secondary signal. This method is known as Current DirectionRecognition.

At the receiver, doubling the lower frequency f₂ produces two signalsf₁, 2f₂ of the same frequency and having phase φ_(f1) and 2φ_(f2). Thequantity φ_(f1)-2φ_(f2) acts as a phase invariant, that is to say it hasone value for the original conductor signal and is 180° shifted for thesignal on the adjacent utility. Therefore it is possible to determineunambiguously if the field being detected is radiated by the targetconductor to which the test signal is applied or by another conductor.

By this method the phase of the two signals at the receiver may becompared to identify the conductor to which the test signal was appliedand the conductor carrying the ground return current.

As the frequencies of the two frequency components f₁, f₂ are separatedby a factor of two this has the disadvantage that the capacitive leakagecurrent of the higher frequency component f₁ is at least twice that ofthe lower frequency component f₂. This causes the frequency componentsf₁, f₂ to experience a different rate of attenuation and phase shiftalong the conductor leading to creepage in the phase differenceφ_(f1)-2φ_(f2) between the frequency components f₁, f₂ generated at thereceiver 7.

Therefore a phase reversal detected at discrete points along the lengthof the buried conductor may be due to either the test signal having‘jumped’ onto a second conductor or due to cumulative incremental phaseshifts between the phases of the frequency components f₁, f₂.

To overcome this potential ambiguity when detecting the test signalalong the target conductor the phase difference between the frequencycomponents f₁, f₂ is compared to a reference phase difference at areference point where the initial phase offset is known. As the measuredphase difference between f₁, f₂ increases, the reference phasedifference is reset so that the phase creepage is tracked. Thistechnique is known as Current Direction Reset.

In an embodiment of the invention the system 1 of FIG. 1 is configuredto reset the current direction measurement according to the method shownin FIG. 6. In step S301 the initial phase difference between f₁ and f₂is calculated by the receiver 7 at a location close enough to thetransmitter 5 where a phase reversal due to creepage will not haveoccurred. In step S303 the phase difference between f₁ and f₂ at thereference location calculated by the receiver 7 is stored in the memory39 of the receiver 7.

The receiver 7 is moved along the buried conductor 3 and at step S305 asubsequent phase difference between f₁ and f₂ is calculated and thephase creepage between the reference location and the current locationis calculated. At step S307 if the phase creepage is below a lowerthreshold value then no action is taken and the receiver 7 is movedfurther along the buried conductor 3 and further phase measurements aremade, returning to step S303. The lower threshold is 5°, preferablybetween 3° and 5° and further preferably 2°.

At step S307 if the phase creepage is above the lower threshold then instep S309 it is determined if the phase creepage is above an upperthreshold. If the phase creepage is determined to be above an upperthreshold then at step S311 the receiver determines that receiver isdetecting a conductor onto which the test signal has ‘jumped’ from thetarget conductor or that the operator has strayed too far from thereference location such that there may have been a large phase creepage.The receiver conveys a warning to the operator that there has been aphase reversal. At step S313 the operator must return to the most recentreference location where it was known that the test signal had notjumped onto a second conductor and recommence detection from there. Themagnitude of the upper threshold is 60°, preferably 80° and furtherpreferably 88°.

If the phase creepage is between the upper and lower thresholds then atstep S315 the receiver sends the “Increment CD F1 Phase” command listedin Table 1 to the transmitter to increment the phase of f₁, therebyreducing the phase difference between f₁ and f₂. In step S317 thetransmitter 5 increments the phase of f1 and the phase differencebetween f₁ and f₂ at this new reference location is stored in thereceiver 7 at step S303. The transmitter 5 sends an “Increment CD F1Phase” acknowledgement to the receiver 7.

Various modifications will be apparent to those in the art and it isdesired to include all such modifications as fall within the scope ofthe accompanying claims.

Aspects of the present invention can be implemented in any convenientform, for example using dedicated hardware, or a mixture of dedicatedhardware and software. The processing apparatuses can comprise anysuitably programmed apparatuses such as a general purpose computer,personal digital assistant, mobile telephone (such as a WAP or3G-compliant phone) and so on. Since the present invention can beimplemented as software, each and every aspect of the present inventionthus encompasses computer software implementable on a programmabledevice. The computer software can be provided to the programmable deviceusing any conventional carrier medium. The carrier medium can comprise atransient carrier medium such as an electrical, optical, microwave,acoustic or radio frequency signal carrying the computer code. Anexample of such a transient medium is a TCP/IP signal carrying computercode over an IP network, such as the Internet. The carrier medium canalso comprise a storage medium for storing processor readable code suchas a floppy disk, hard disk, CD ROM, magnetic tape device or solid statememory device.

The many features and advantages of the invention are apparent from thedetailed specification, and, thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, and,accordingly, all suitable modifications and equivalents may be resortedto that fall within the scope of the invention.

1. A system for detecting a buried conductor, the system comprising:means for producing an alternating test current in said buriedconductor; means for detecting an electromagnetic field produced by thetest current in said buried conductor; and means for providing acommunication link between the means for producing an alternating testcurrent and the means for detecting an elecromagnetic field; wherein themeans for producing an alternating test current and the means fordetecting an electromagnetic field are configured for the means fordetecting an electromagnetic field to set, via the communication link,characteristics of the test current produced by the means for producingan alternating test current in response to the electromagnetic fielddetected at the means for detecting an electromagnetic field withoutintervention from an operator of the system.
 2. A system as claimed inclaim 1, wherein the means for producing an alternating test current andthe means for detecting an electromagnetic field are configured for themeans for detecting an electromagnetic field to set characteristics ofthe test current produced by the means for producing an alternating testcurrent by: the means for detecting an electromagnetic field beingconfigured to determine desirable characteristics of the test currentand to transmit the desirable characteristics of the test current to themeans for producing an alternating test current; and the means forproducing an alternating test current being configured to receive thedesirable characteristics of the test current and set thecharacteristics of the test current to correspond to the desirablecharacteristics determined by the means for detecting an electromagneticfield.
 3. A system as claimed in claim 1, wherein the characteristics ofthe test current set by the means for detecting an electromagnetic fieldcomprise power and/or frequency of the test current.
 4. A system asclaimed in claim 1, wherein the communication link between the means forproducing an alternating test current and the means for detecting anelectromagnetic field is provided by a transceiver at each of the meansfor producing an alternating test current and the means for detecting anelectromagnetic field.
 5. A system as claimed in claim 1, wherein thecommunication link between the means for producing an alternating testcurrent and the means for detecting an electromagnetic field is a duplexor half-duplex communication link.
 6. A system as claimed in claim 1,wherein the means for detecting an electromagnetic field comprises meansfor calculating the signal to noise ratio, SNR, of the detectedelectromagnetic field at a specified frequency range.
 7. A system asclaimed in claim 6, wherein the means for detecting an electromagneticfield is configured to alter the frequency of the alternating testcurrent, if the SNR is below a lower threshold, by incrementing ordecrementing the frequency by a predetermined amount.
 8. A system asclaimed in claim 1, wherein the means for detecting an electromagneticfield is configured to set the characteristics of the test current basedon the complex impedance of the ground at the means for producing analternating test current.
 9. A system as claimed in claim 1, wherein thecommunication link between the means for producing an alternating testcurrent and the means for detecting an electromagnetic field is awireless communication link.
 10. A system as claimed in claim 1, whereinthe test current in said buried conductor is produced by the means forproducing an alternating test current by means of an output module, theoutput module either radiating an electromagnetic field to induce thetest current in said buried conductor, being directly connected to apart of said buried conductor or clamping the output module around saidburied conductor.
 11. A system as claimed in claim 1, furthercomprising: analogue to digital converters to convert the field strengthsignals into digital signals; and a digital signal processor arranged toprocess the digital signals and to isolate signals of predeterminedfrequency bands, wherein the means for detecting an electromagneticfield includes a plurality of antennas for detecting the electromagneticfield produced by the test current in said buried conductor, and whereineach antenna outputs a field strength signal representative of theelectromagnetic field at the antenna.
 12. A method of detecting a buriedconductor, the method comprising: providing a transmitter for producingan alternating test current in said buried conductor; providing areceiver for detecting an electromagnetic field produced by the testcurrent in said buried conductor; providing a communication link betweenthe receiver and the transmitter; and the receiver settingcharacteristics of the test current produced by the transmitter via thecommunication link between the receiver and transmitter withoutintervention from an operator of the system.
 13. A method as claimed inclaim 12, wherein the receiver sets the characteristics of the testcurrent produced by the transmitter by: determining desirablecharacteristics of the test current at the receiver; transmitting thedesirable characteristics of the test current from the transmitter;receiving the desirable characteristics of the test current at thereceiver; and setting at the transmitter the characteristics of the testcurrent to correspond to the desirable characteristics determined at thereceiver.
 14. A method as claimed in claim 12, wherein the communicationlink between the receiver and transmitter is a duplex or half-duplexcommunication link.
 15. A method as claimed in claim 12, wherein thereceiver calculates the signal to noise ratio, SNR, of the detectedelectromagnetic field at a specified frequency range.
 16. A method asclaimed in claim 15, wherein the receiver alters the frequency of thealternating test current, if the SNR is below a lower threshold, byincrementing or decrementing the frequency by a predetermined amount.17. A method as claimed in claim 12, wherein the characteristics of thetest current are set by the receiver based on the complex impedance ofthe ground at the transmitter.
 18. A method as claimed in claim 12,wherein the communication link between the transmitter and receiver is awireless communication link.
 19. A method as claimed in claim 12,wherein the test current in said buried conductor is produced by thetransmitter by means of an output module, the output module eitherradiating an electromagnetic field to induce the test current in saidburied conductor, being directly connected to a part of said buriedconductor or clamping the output module around said buried conductor.20. A method as claimed in claim 12, the receiver comprising a pluralityof antennas for detecting the electromagnetic field produced by the testcurrent in said buried conductor.
 21. A method as claimed in claim 20,further comprising: analogue to digital converters to convert the fieldstrength signals into digital signals; and a digital signal processorarranged to process the digital signals and to isolate signals ofpredetermined frequency bands, wherein each antenna outputs a fieldstrength signal representative of the electromagnetic field at theantenna.
 22. A system for detecting a buried conductor, the systemcomprising: a transmitter for producing an alternating test current insaid buried conductor; a receiver for detecting an electromagnetic fieldproduced by the test current in said buried conductor; and means forproviding a communication link between the receiver and transmitter;wherein the transmitter and receiver are configured for the receiver toset, via the communication link between the receiver and transmitter,characteristics of the test current produced by the transmitter inresponse to the electromagnetic field detected at the receiver withoutintervention from an operator of the system.